Apparatus and method for frequency modulation for sampled amplitude signal generating system

ABSTRACT

A memory contains data in a plurality of discrete locations and is addressed at a rate that depends upon desired spacing between data from various locations as it is sequentially read from the memory. In a specific embodiment the data constitutes amplitude values of a complex wave form of a type produced by a musical instrument at equally spaced points in time along an axis of the wave form. The memory is addressed at any one of a plurality of rates selected in accordance with a musical note to be played. Selection of a particular note results in a repetitive read out from the memory of groups of amplitude samples that collectively represent a wave shape. The group repetition rate represents the frequency of the desired musical tone and is determined by a control number that is unique for each output frequency. The control number periodically increases a number stored in the memory address register by the value of the control number so as to identify appropriate data addresses in the memory. Having selected a unique control number and therefore a unique output signal frequency, the latter is modulated by changing the control number during the repetitive generation of groups of amplitude samples. The same magnitude of small change in the control number for a plurality of frequencies to be generated will achieve an ensemble effect for octave decoupling. A relatively small magnitude repetitive variation of the control number, as groups of amplitude samples are repetitively read from the memory, will provide a tremulant effect. Other types of variation of the control number will achieve other types of frequency modulation.

United States Patent [191 Deutsch [451 Feb. 26, 1974 1 APPARATUS ANDMETl-IOD FOR FREQUENCY MODULATION FOR SAMPLED AMPLITUDE SIGNAL GENERATING SYSTEM [75] Inventor: Ralph Deutsch, Sherman Oaks,

Calif.

[73] Assignee: North American Rockwell Corporation, El Segundo, Calif.

[22] Filed: Dec. 6, 1 971 21 Appl. No.: 205,092

521 US. Cl. 8471.24, 84/DIG..4

[51] Int. Cl. G10h 1/02 [58] ,Field of Search. 84/1.0l, 1.24, 1.25, 1.26,

[56] 7 References Cited UNITED STATES PATENTS 2,905,905 9/1959 George 84/1.0l X

2,989,887 6/1961 Markowitz... 84/l.24

3,007,361 l'l/l96l Wayne.... 84/l.0l

3,157,725 ll/1964 Wayne...., 84/l.01 X

3,288,904 11/1966 George 84/l.24 X

3,288,907 ll/l966 George 84/125 X 3,474,182 10/1969 Destelle... 84/125 3,490,327 l/l970 Volpe 84/l.0l X

3,516,318 6/1970 Wayne....- 84 /l.0l

3,515,792 6/1970 Deutsch... 84/124 X 3,610,799 10/1971 Watson 84/l.0l

Primary ExaminerStephen .l. Tomsky Assistant Examiner-U. Weldon Attorney, Agent, or FirmH. Fredrick l-lamann; L. Lee l-lumphries [5 7] ABSTRACT A memory contains data in a plurality of discrete locations and is addressed ,at a rate that depends upon desired spacing between data from various locations as it is sequentially read from the memory. In a specific embodiment the data constitutes amplitude values of a complex wave form of a type produced by a musical instrument at equally spaced points in time along an axis of the wave form. The memory is addressed at any one of a plurality of rates selected in accordance with a musical note to be played. Selection of a particularnote results in a repetitive read out from the memory of groups of amplitude samples that collectively represent a wave shape. The group repetition rate represents the frequency of the desired musical tone and is determined by a control number'that is unique for each output frequency. The control number periodically increases a number stored in the memory address register by the value of the control number so as to identify appropriate data addresses in the memory. Having selected a unique control number and therefore a unique output signal frequency, the latter is modulated by changing the control number during the repetitive generation of groups of amplitude samples. The same magnitude of small change in the control number for a plurality of frequencies to be generated will achieve an ensemble effect for octave decoupling. A relatively small magnitude repetitive variation of the control number, as groups of amplitude samples are repetitively read from the memory, will provide a tremulant effect. Other types of variation of the control number will achieve other types of frequency modulation.

13 Claims, 3 Drawing Figures mom MULTIPLEXER 28 NUMBER SAMPLING SLECTOR CLOCK TONE GENERATOR 27 l I I -"ir r r "fi L PMSEE #g A nintiess GATE l R RE I some 3s mom 'ADDRESS. GENERATOR ASSIGIIAENT necom-zn \as v MEMORY PATENTEBFEBZS m4 SHEET 1 (IF 3 m T w mm w m w m.. ||l|||.Pl .llll. 4 l n 2 m I H m mm VI 8 EU R E W o R E 0 EL m 0 WE S u p m IT! SS r I I I IIL STEP SIZE CONTROL FIG.

ATTORNEY 1 i APPARATUS AND METHOD FOR FREQUENCY MODULATION FOR SAMPLED AMPLITUDE SIGNAL GENERATING SYSTEM This application discloses an improvement in the system of a co-pending application of George A. Watson, Ser. No. 875,178, filed on Nov. 10, 1969, now US. Pat. No. 3,639,913 for Method and Apparatus For Addressing A Memory At Selectively Controlled Rates, which is assigned to the assignee of the present application and incorporated by this reference as though fully set forth herein.

BACKGROUND OF THE INVENTION 1. Field of the lnvention The present invention relates to methods and apparatus for frequency modulating cyclically repetitive signals that include repetitive groups of amplitude samples of a wave shape, and more particularly concerns frequency modulation of signals representing musical tones produced by an electrical musical instrument.

2. Description of Prior Art Electrical musical instruments generally provide various musical tones at frequencies that are precisely related according to mathematically predetermined musical scales. The average human ear can distinguish 1,400 discrete frequencies. However, in the equal tempered scale covering the hearing range of 16 to 16,000 cycles, there are only- 120 discrete tones. In otherwords, musical tones may be said to be quantized, in that only certain discrete frequencies are allowed and others ruled out. Thus, a note in one octave has a frequency that is exactly one half the frequency of a corresponding note in a higher octave whereby the second harmonic of the lower note is exactly in tune with the first harmonic of the higher note. Although this relation is mathematically precise, it does not accurately simulate real instruments. For example, a large part of the subjective total effect of a pipe organ is that each of the pipes is not exactly at the precise musical frequency. These inherently detuned pipes add to each other in a very pleasant sounding manner to produce what is known as an ensemble effect. This effect is probably best known in connection with a symphony orchestra. As an example, a group of violinists playing in unison has such an ensemble effect, whereas a single violin amplified to the volume achieved by the group lacks the ensemble sound. The effect is directly related to the relatively low frequency beats heard between tone,

sources that are slightly out of tune relative to each other.

Early versions of electronic organs attempted to imitate the ensemble effect of a rank of pipes by using an individual oscillator for each note. In such an arrangement, inherent drift and lack of precision of initial tuning will cause each of these oscillators to be slightly off its mathematically determined musical frequency. Al-

though such individual oscillator systems are effective for achieving the ensembleeffect, they require an expensive system. Moreover, unless exceeding high quality oscillators are employed, the problem of keeping the instrument in tune is difficult.

A number of present day electronic organs employ a set of 12 oscillators to generate the highest octave of frequencies for the instrument and synthesize other frequencies. Lower frequencies are generated by a chain of divide by two or flip flop frequency dividers, there being one chain for each of the 12 oscillators. Such organs are economical, but lack the subjective pleasure of the ensemble tone quality (as when several notes are played simultaneously). This characteristic of the frequency divider electronic organ is readily detectable by a listener as a characteristic tone of the instrument.

Various versions of frequency divider electronic organs are presently on the market or in development. An objective of at least some of these developments is to produce a frequency synthesizing system that will replace the twelve oscillators used by the frequency divider system. An advantage of the frequency synthesizer system is that the organ will not, require tuning of individual oscillators and will not require tuning in the sense that the required equal temperament of the musical scale cannot change. Only absolute pitch can change in such a system. However, the frequency synthesizer organ system merely comprises the input to a conventional frequency divider chain and accordingly the organ still lacks the ensemble effect.

A recently developed and unique type of frequency synthesizer organ is the digital organ invented by Ralph Deutsch and described in U. S. Pat. No. 3,515,792. In the system of the digital organ of this patent amplitude samples of a complex waveform are stored in a memory and groups of these samples are read out and then combined to provide the desired musical tone. The read out repetition rate is determined by a control signal that is selected by depressing a key on the instrument. In the Deutsch patent this control signal is a pulse repetition rate that steps a memory addressing register.

Modified forms of the Deutsch Digital Organ are described in the above-identified co-pending application of George A. Watson for Method and Apparatus For Addressing A Memory At Selectively Controlled Rates, Ser. No. 875,178 filed on Nov. 10, 1969, now US. Pat. No. 3,639,913 and assigned to the assignee of the present invention, and in U. S. Pat. No. 3,610,799 of George A. Watson. In these arrangements of Watson, the control signal that determines repetition rate of groups of amplitude samples that are read from the memory is a number stored in a phase angle register. In

both the Deutsch Patent and in the improvements described by Watson in the above-identified patent application and patent, the control signals, whether the repetition rate of Deutsch or the phase angle number of Watson, are precisely calculated according to abovestated mathematical relations between the notes of the equal temperament musical scale. In'this equal temperament scale the octave is divided into 12 equal intervals called tempered half tones. A semi tone, or half tone, is the frequency ratio between any two tones whose frequency ratio is the twelfth root of two. Thus, without any further detuning the ensemble effect is still lacking.

Accordingly, it is an object of the present invention to provide methods and apparatus for modulating electrical signals that represent musical tones, and, in one arrangement to achieve an ensemble effect in a frequency synthesizing instrument.

SUMMARY OF THE INVENTION groups of representations of amplitude samples that collectively delineate the wave shape of the desired sig-. nal are generated at a group repetition rate that is determined by the control signal. Modulation is achieved by modulating the parameter of said control signal as it is employed to produce the periodically repetitive output signal. More specifically, a memory containing representations of amplitude samples of a complex wave form is addressed at a cyclic rate that determines the resulting signal frequency and which rate is' itself determined by a control signal. Modulation is achieved by varying the control signal as the memory is being cyclically addressed. The system achieves octave de- 7 coupling or detuning of tones that are mutually spaced by integral multiples of an octave, or other numbers of half tones, by changing the frequency of each of the signals of at least a group of the signals by a selected amount.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3 illustrates one tone generator of the instru-- ment of FIG. 2, showing application of the modulation system of the present invention, and

DETAILED DESCRIPTION The present invention is generally applicable to a variety of different systems in which groups of signals are repetitively read or extracted from a memory at a selected or nominal group repetition rate. According to the present invention, this selected or nominal group repetition rate may be modulated in a variety of different modes. Because the invention has been initially embodied in an electrical musical instrument, it will be described in a preferred embodiment as incorporated. in such an instrument. In particular, a primary application of the invention is in the production of an ensemble effeet, or octave detuning of an electronic organ.

In the frequency divider organ and in the frequency synthesizer organ, two corresponding notes in different octaves such as the note C in one octave and the note C in an adjacent octave are exactly in phase in the sense that the second harmonic of the lower note is exactly in tune with the first harmonic of the higher note.

For example, let

f lower frequency f higher frequency, and

f 2f 1, where the notes are an octave apart.

If the two notes are sounded simultaneously, the beat frequency,f,, is

f1, lfz f1l=l f1 fll This is the situation that occurs in a frequency divider or frequency synthesizer organ and indicates the total lack of ensemble effect.

According to an aspect of the present invention, we may add a small magnitude difference frequency, d, to each of f, and f The beat frequency now is It will be noted that adding the same freque'ncy deviation d to each of two notes that are separated by an octave produces a beat note of d. Nowconsider notes several octaves apart in which case Adding d to both f and f produces the beat frequency Thus as the number of octaves separating the two frequencies increases, the beat frequency produced by adding d to each frequency also increases. This effect of an increase of beat frequency with an increase of octave separation is closely analogous to that which actually occurs in real organ pipes. V

' Review of the above will readily indicate the same results will be achieved by subtracting the quantity d, rather than adding it to the several frequencies being compared.

Preferably the magnitude of the frequency deviation d is about one half Hertz. Detuning by an amount in the order of 0.25 to 1.00 Hertz produces a pleasant'ensemble sound. Detuning of about I to 4 Hertz will produce a celeste sound which is also pleasant. Beats of about 5 to 7 Hertz may produce a tremulant effect which adds a warmth to the tone. However, beats above 7 Hertz are generally considered to be unpleasant and usually judged to indicate an undesired out-of-tune condition.

Application of the above concepts to a cyclically repetitive memory read out system will now be described in connectionwith FIG. 1. Illustrated in this figure are certain portions of a cyclically repetitive memory and dressing system embodying apparatus for achieving there appears upon output line 14 a group of signals comprising respective ones of the signals stored in the memory. When the addresser gets to the last memory positiomor to a final position in its cycle, it automatically recycles itself to the firstposition'and again begins its step by step addressing of memory locations. The

time required for the addresser to read each memory location is one cycle or period. Accordingly, the group of signals on output lead 14 is repeated at a group repetition rate equal to the cyclic rate of the addresser.

The memory may be any one of a number of well known types, whether analog or digital. If digital, the memory may be a conventional read-write memory of a type wherein information may be readily written into and read from the memory by well known electronic circuitry. The memory may be of the permanent memory type, also known as a read-only memory, where information is stored by specific connections of wiring or by matrices of intersecting row and column conductors selectively coupled at such intersections by magnetic cores, diodes, or transistors. As an example, the memory may comprise a diode array of the type described in U.S. Pat. No. 3,377,513 to Ashby et al., such memory comprising a micro-electronic integrated circuit wherein a large number of diodes are arranged in a matrix of columns and rows on a single micro-electronic chip.

Where the present invention is applied to a digital organ, memory may comprise the wave shape memory of a system such as shown in the Deutsch Pat. No. 3,515,792, or the read-only memory described in U.S. Pat. No. 3,6l0,799, or in the above-identified copending application of George A. Watson for Method and Apparatus For Addressing A Memory At Selectively Controlled Rates. As explained in detail in the Deutsch patent and in the patent and application of Watson, the memory stores samples of amplitude values of a musical wave form which samples are taken at equally spaced points in time along an axis of the wave form. The actual value of each amplitude may be stored in the particular memory location in digital form as a digital word of, for example, seven bits and sign. Alternativey, only amplitude increments need be stored in such digital form, such amplitude increments being the difference in magnitude between two successive amplitude samples. Accordingly the terms amplitude samples, sampled amplitude, or famplitude as applied to information stored in the memory or. representing a complex wave shape will be deemed to include both actual magnitudes of the information and/or such amplitude increments.

Although the invention is described herein as applied to the digital organ systems such as the Deutsch and Watson patents referred to above, it will be readily appreciated that it is not limited to systems employing digital representations of sampled amplitudes. Principles of the invention may also be applied to instruments and systems that delineate complex wave shapes of musical tones by means of other types of representations of sampled amplitudes. Such other representations may include various well known forms of analog arrangements, such as voltage, current, electrical charge, and the like. Therefore the term amplitude or amplitude sample or sampled amplitude as applied to information stored in a memory or representing a complex wave shape shall be deemed to include the various types of representations of such sample amplitudes, whether digital, analog, or other. I

Groups of amplitude sample representations collectively delineating the wave shape of the complex musical tone appear on output line 14 at a group repetition rate directly related to a selected frequency. These groups of amplitude samples are combined and modified by various accumulators, attack and decay circuits and the like as more particularly described in the patent to Deutsch U.S. Pat. No. 3,515,792, in the aboveidentified patent and application of George Watson, or as described in a co-pending application of Ralph Deutsch for Wave Shape Smoother Ser. No. 204,807, filed on Dec. 6, 1971, and assigned to the assignee of the present application. Such co-pending application of Ralph Deutsch for Wave Shape Smoother is incorporated in this disclosure by this reference as though fully set forth herein. 7

Cyclic addresser 12 is analogous to a ring counter that steps from one count to the next upon each occurrence of aclock pulse provided from a suitable clock source. The arrangement is such that the addresser does not address and read a different memory location at each of its counts. Rather, the addresser will read a different address of the memory only when it counts to particular counts that are separated from each other by preselected amounts. For example, consider a decimal analogy in which memory 10 contains ten locations and accordingly ten digital words. The addresser may be arranged to count consecutively from one through one hundred and is connected to address a new memory location only upon each tenth count although it will read upon each count or clock pulse. Accordingly when the addresser reaches its count of ten a first word W will be read from the memory, The second word W is read when the addresser reaches the count twenty, and so on, to read the tenth and last word upon reaching the count of one hundred whereupon the addresser starts counting anew.

The number by which the addresser counter advances upon each clock pulse may be termed a step size. Thus the addresser may be set to advance by a single unit upon each clock pulse so that the step size is one. If the clock pulse is fed to the second lowest order of the addresser counter rather than to the first order, the addresser will step by a count of two upon each clock pulse. Similarly, if the clock pulse is fed to the next order, the addresser may step by a count of four. Obviously many different arrangements are available to control the step size of the addresser. One such ar-' rangement is shown in the above identified patent and application to George A. Watson, and will be more particularly described hereinafter in connection with FIG. 3.

Accordingly, there is provideda step size circuit 16 that produces a control signal on line 18 that is effective to vary the step size of the addresser. The control signal provided by the step size circuit 16 has a parameter such as magnitude, repetition rate, pulse width or the like that is'varied in accordance with a frequency selector signal provided as a primary input on line 20 to the step size circuit 16. In a sampled amplitude organ for example, the frequency selector signal on line 20 would be chosen by the operation of a given key of the instrument that accordingly will control a basic or nominal group repetition rate of the amplitude samples read from the memory and therefore control the frequency of the resulting musical signal.

In a musical instrument, the frequency selector signal on line 20 is calculated or chosen from a set of numbers that are one half tone apart, so as to selectively generate tones whose frequency ratio is the twelfth root of two. The number of discrete signals thus available on control line 18 is the number of different frequencies to be available from the instrument, there nominally being at least one unique frequency selector signal available on line 20 for each output frequency of the instrument. Of course different octaves may be obtained byshifting the number in addresser 12 to the right (divide by two) or to the left (multiply by' two).

According to the present invention any frequency selected by any of the set of frequency selector signals on line 20 can be modulated in various modes without any effect upon amplitude and without interfering with the basic selection of notes of the scale of equal temperament. To this end a frequency modulation or step size control circuit 22 is provided to change the step size that is nominally controlled by the frequency selector signal on line 20. The modulation control provided by circuit 22 superimposes a modulating variation upon the control signal 18.

A digital organ embodying the present invention may be of the type more particularly described in the aboveidentified co-pending application and patent of George A. Watson. Briefly, as illustrated in FIG. 2 hereof the overall arrangement of such a digital organ embodies a multiplexer 24 that provides a series of output signals on a line 25, each of which occurs in a unique specifically allocated time slot of each multiplexer cycle. As the operator actuates a given key or pedal, or some combination of keys and pedals of the instrument, the arrangement scans each key and pedal once during each multiplexer cycle and produces a pulse or no pulse at a particular time slot allocated to a given key depending upon whether such key or pedal has been actuated. The multiplexed signal on line 25 is fed to a generator assignment logic circuit 26 which feeds the pulses representing actuated keys or pedals to individual ones of a plurality of tone generating circuits 27a through 27n. Preferably there are twelve such tone generating circuits since it is highly unlikely that more than 12 notes and pedals will be actuated simultaneously. The function of the generator assignment logic is to direct a signal from the multiplexer representing actuation of a given key or pedal to a tone generator that is not already engaged in receiving a signal and producing a tone therefrom.

Each tone generator shares the operation of a common phase angle number selector 28 that stores or repetitively calculates a set of distinct and different numbers by multiplication by the twelfth root of two. Each such number identifies the phase angle of stored sample points of the complex wave form for respective note frequencies in the entire range of frequencies capable of being generated by the organ. Details of such calculation and/or storage, together with circuitry therefore are set forth in the above-mentioned patent and application of- Watson.

The tone generators address a memory 31 to achieve read out therefrom of groups of stored sampled amplitudes, at group repetition rates corresponding to the particular note or notes that are actuated. These repeated groups of samples from the memory are accumulated, shaped, combined and converted to an audio signal in circuitry collectively indicated at 29 and described more particularly in the above-identified application and patent of George A. Watson. As previously indicated, the memory 31 may be one of many different types. It may comprise a number of memories or actually a number of memory sections, each memory or each section of the memory storing amplitude samples of a different complex wave form. Each memory or each memory section may be shared by all of the tone generators.

Illustrated within the dotted lines of FIG. 3 is a single one of the tone generator circuits 27a through 27n. Since each of the tone generator circuits is identical to all of the others, details of only one are shown. When the generator assignment logic 26 (FIG. 2) determines that a particular tone generator is claimed (available for reception of the next note identified in the multiplexed signal), a gate 30 is opened to allow a number corresponding to the particular note or actuated key that is to be assigned to this tone generator to be fed to a phase angle register 32.

Phase angle register 32 feeds the ,number stored therein to a sample point address register 34 and upon each pulse received from a sampling clock, augments the number stored in the sample point address register 34, by the number in the phase angle register. In other words, the number stored in the phase angle register is analogous to the frequency control signal that appears on line 18 of FIG. l'and also is analogous to the step size described in connection with FIG. 1. The number stored in the phase angle register 32 is but a small fraction of the total number to which the sample point address register may advance. As the sample point address register advances, its count is fed through gating 35 to be decoded in an address decoder 36 which then time slot of the multiplexer so that each number will be uniquely timed with and allocated to a specific fre-.

quency chosen when a specific one of the note keys or pedals of the instrument is actuated. Since the phase angle numbers that are selected in circuit 28 have a predetermined mathematical relation according to the particular algorithm of the calculation, it will be seen that different phase angle numbers representing corresponding notes will be so precisely related as to eliminate the desired ensemble effect.-

According to principles of the present invention, the resulting frequency of the signal derived from the repetitive memory read out, which frequency is nominally determined by the phase angle number in phase angle register 32, is modulated by changing the phase angle number independently of the nominal number selected by the particular key or pedal that is actuated. Althoughthe phase angle number is selected from a precalculated or stored set of numbers each uniquely related to a step in the equal temperament musical scale, the selected number is changed by an amount that causes it to have a magnitude that is not contained within this set. In other words, the note is detuned. Since this one phase angle registermay control a plurality of different notes in the described multiplexed note selection scheme, a frequency deviation is modulated upon each note if desired. Further, each of several tone generators of a given instrument will achieve the described frequency modulation. Thus the ensemble effect is produced.

In an exemplary embodiment of the invention, the V phase angle register stores, in binary form, a number having fourteen bits, (the first bit being of highest order and the fourteenth bit of lowest order or significance). The arrangement is such that a change in the eleventh bit of the number stored in the phase angle register, changing such bit from a one to a zero, or from a zero to a one, will achieve an amount of frequency deviation of the resulting musical tone of about one half Hertz. Accordingly, for octave decoupling, one bit in the eleventh order of the fourteen bit phase angle number is fixed whenever an ensemble switch 46 is closed. In effect, circuit 42 merely feeds a signal from circuit 44 to a given stage (a given flip-flop) of the stages or orders of the register 32 so as to change the state of such stage, and, thereby to change the bit in that order of the number contained in the register. It is well known that the state of one stage of the register can be changed (to thereby change one bit of the number contained in the register) merely by feeding a suitable signal to the stage via a switch such as switch 46.

Increasing the phase angle register results in making each tone slightly sharp. It will be readily appreciated that the frequency deviation could be achieved by making each tone slightly flat, as by subtracting one bit from the eleventh order of the fourteen bit phase angle number. Moreover addition or subtraction may be performed either with or without carry. If performed without a carry, a greater randomness of the deviation is achieved to more closely simulate the actual organ pipe. Of course, for a random change of one bit without carry it is enough to simply wire the specific order of the phase angle number to a given condition (via the ensemble switch 46) so that it will remain a one, or remain a zero and thus achieve a deviation from its nominal value. Thus, as given phase angle number is fed to the phase angle register, each of the several orders (stages) of the register will change to reflect the digit in such order except that the specific order that is wired to a given condition will remain in such condition, thus achieving a random variation of the phase angle number. It will be readily understood that a phase angle number having 14 orders and the described change in the eleventh order are exemplary only, since phase angle numbers of other orders and, concomitantly, modulation by change of a different order of such number may be readily employed. In any event, with the described l4 bit number it is found that a change in an order of greater significance achieves too much detuning so as to provide a subjectively unpleasant sound, whereas a change in an order of less significance, may not provide sufficient detuning for an optimum ensemble effect. As indicated above, a change sufficient to provide a frequency deviation of one half Hertz is optimum for octave decoupling. Of course, the phase angle register must have a sufficient number of stages or orders so as to enable the desired one half Hertz resolution.

The amount of octave decoupling should vary' according to particular voices involved. Pure tones such as flutes need a greater frequency deviation than complex tones such as reeds in order to achieve optimum octave decoupling. Of course, all of this is subjective to the listener. Accordingly if reed voices are employed, the bit change may be directed to an order of less significance and if flute voices, the bit change may be directed to an order of higher significance of the phase angle register. Although change of a single bit is most convenient, the actual magnitude of change (and the number of bits affected) may be selected as deemed appropriate for any particular system and desiderata. Suitable switches (not shown) may be connected with stops of the respective voices so asto affect different magnitudes of frequency deviation when different voices are selected.

In the arrangement shown in FIG. 3 the magnitude of the change in the phase angle number that is achieved may be controlled by the output of a pseudo-random 10 tional and well known type that will provide a series of ones and zeros, at the clock rate for example, in a pseudo-random variation, but in such a manner as to have an average sum of ones and zeros of substantially zero.

A tremulant effect may be obtained by periodically varying the phase angle number under control of a tremulant commanding square wave for example. Thus, the phase angle number may be cyclically varied by a relatively small amount, or deviated by a relatively small amount from its nominal value, to produce a cyclically deviating frequency. A tremulant arrangement may provide a frequency change of about 11 to 23 cents in each direction in .the resulting musical tone that is derived from the repetitive groups of amplitude samples read from the memory. Thus for a selected time, such as a predetermined number of cycles of the nominal tone, its frequency is raised and, immediately thereafter, its frequency is lowered by a like amount for an equal or similar time. The magnitude of this frequency deviation may be varied by causing the change in phase angle number to occur in a different .order. The periodic rate of frequency deviation is adjusted to be in the range of 5 to 7 Hertz.

lf deemed necessary or desirable, the bit position of I the phase angle number that is changed may be varied from octave to octave. Different amounts of tremulant may be available for different voices or different octaves. Y

It should be noted that all of the frequency modulation arrangements described above are pure frequency modulation. That is, the desired methods and apparatus achieve solely change in frequency. No amplitude variation is in any way caused by this frequency modulation. This is quite different from analog circuit frequency changes where impedance of various components changes with frequency to thereby affect signal amplitude.

The described arrangements employ control signals taking a digital form, these control signals being the number contained in the phase angle register. In the arrangement described in the patent to Deutsch Pat. No. 3,515,792, the control signals actually comprise pulse trains, each having a repetition rate that is directly related to the frequency of the musical tone to be synthesized from the read out of amplitude samples. Itwill be seen that whether the significant parameter of the control signal be magnitude, as in FIG. 3, or repetition rate as in the Deutsch patent, or some other signal characteristic, the control of such parameter to cause it to deviate from a nominal value will achieve the desired frequency modulation.

There have been described methods and apparatus for obtaining a variety of types of frequency variation wherein frequency may be modulated linearly with time or according to some other time program, wherein the frequency may be deviated by fixed preselected amounts or periodically deviated about a nominal frequency, all with no effect upon amplitude.

The foregoing detailed description is to be clearly understood as given byway of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.

I claim:

1. The method of changing first and second notes thathave nominal frequencies of different steps in a musical scale comprising the steps of generating a first digital control signal having a magnitude that directly controls frequency of a first note in said musical scale to be generated thereby, generating said first note from said first digital control signal, generating a second digital control signal having a magnitude that directly controls the frequency of a second note in said musical scale to be generated thereby,

generating a second note from said second digital control signal, and changing at least one of said digital control signals so as to effect a relatively small change in the frequency of the. note. generated thereby.

2. The methodof claim 1 wherein said first and second notes are corresponding notes of different octaves, and wherein both of said control signals are changed in substantially the same way so as to introduce a small beat frequency into the combination of said notes.

3. The method of claim 1 wherein each said control 2 signal is a digital representation having a nominal magnitude that is uniquely related to an individual note frequency.

4. The method of claim 1' wherein each said control signal is a representation of a magnitude selected from a set of magnitudes each of which is uniquely related to a different step in said musical scale, and wherein said step of changing one of said control signals comprises changing said representations to magnitudes not contained within said set of magnitudes.

5. The method of claim 2 wherein said step of changing at least one of said control signals comprises the step of changing both of said control signals by the same amount whereby the frequencies of said first and second notes are both changed by the same amount.

6. A frequency modulation system for an electrical musical instrument comprising means for generating a frequency control signal having a parameter directly related to frequency of a musical signal to be generated therefrom, means responsive to said frequency control signal for generating repetitive groups of amplitude sample representations collectively delineating a wave shape of a musical note, the repetition rate of said groups being directly controlled by said control signal, means for selecting a value of said parameter that uniquely defines a desired nominal frequency of a musical note, modulating means for changing the selected value of said control signal parameter, and

means for converting said groups of amplitude sam-,

ple representations to a musical signal of said nominal frequency as modulated by said modulating means.

7. The system of claim 6 wherein said control signal is a digital representation of a number selected from a set of numbers, wherein said means for generating groups of amplitude sample representations comprises an address register, a control number register, and means for repetitively advancing the address register by the number in thecontrol number register, and wherein said modulating means comprises means for changing the number in said control number register to a number not contained in said set of numbers.

8. The frequency modulation system of claim 6 wherein said means for generating groups of amplitude sample representations comprises a memory storing representations that collectively delineate such-wave representations at a group repetition rate that defines the nominal frequency of a wave shape to be generated, wherein said means for generating a frequency control signal comprises a number storage device and means for introducing into said number storage device a number that identifies a nominal musical signal to be generated, and wherein said modulating means comprises means for changing at least one digit of said number in said number storage device.

9. The frequency modulation system of claim 8 .wherein said modulating means comprises means for changing the digit of a single orde'riof the number in said number storage device by a selected amount for any one of a plurality of numbers-that may be contained in said number storage device.

10. A musical tone generation system comprising a memory containing representations of sampled amplitudes of a wave form of a musical note,

memory address means for repetitively reading said representations from said memory to providean output comprising repetitive groups of said representations, said groups having repetition rate corresponding to the frequencyof'the musical tone to the frequency of the musical tone to be generated thereby, a number storage device and meansfor repetitively augmenting said memory address means by the amount of the number stored in said number storage device, thereby to control the repetition rate of groups of representations read from said memory, means for selecting the frequency of a note to be generated,

means responsive to said note frequency selecting means for introducing into said number storage de vice a number selected from a set of numbers that represent notes of a musical scale, and

frequency modulation means for changing the number in said numberstorage device as representations are being read from said memory.

11. The tone generation system of claim 10 wherein said frequency modulation means comprises means for changing the digit stored in a preselected order of said number storage device.

12. In a musical tone generation system comprising a memory containing representations of sampled amplitudes of a wave form of a musical note, memory address register means for repetitively reading said representations from said memory to provide an output comprising repetitive groups of said representations, said groups having a repetition rate corresponding to the frequency of the musical tone to be generated thereby, address rate control means for controlling said address register to thereby control the read out repetition rate of said groups read from said storage memory, and manually selectable means for controlling said address rate control means so as to select a desired musical tone of a musical scale, the improvement comprising means for modulating said address. rate control means so as to modulate the frequency of the selected musical tone. 13. The system of claim 12 wherein said address rate control means is a number storage register, and

whereinsaid modulating means comprises means for changing the digit in one order of low significance of said number storage register. 

1. The method of changing first and second notes that have nominal frequencies of different steps in a musical scale comprising the steps of generating a first digital control signal having a magnitude that directly controls frequency of a first note in said musical scale to be generated thereby, generating said first note from said first digital control signal, generating a second digital control signal having a magnitude that directly controls the frequency of a second note in said musical scale to be generated thereby, generating a second note from said second digital control signal, and changing at least one of said digital control signals so as to effect a relatively small change in the frequency of the note generated thereby.
 2. The method of claim 1 wherein said first and second notes are corresponding notes of different octaves, and wherein both of said control signals are changed in substantially the same way so as to introduce a small beat frequency into the combination of said notes.
 3. The method of claim 1 wherein each said control signal is a digital representation having a nominal magnitude that is uniquely related to an individual note frequency.
 4. The method of claim 1 wherein each said control signal is a representation of a magnitude selected from a set of magnitudes each of which is uniquely related to a different step in said musical scale, and wherein said step of changing one of said control signals comprises changing said representations to magnitudes not contained within said set of magnitudes.
 5. The method of claim 2 wherein said step of changing at least one of said control signals comprises the step of changing both of said control signals by the same amount whereby the frequencies of said first and second notes are both changed by the same amount.
 6. A frequency modulation system for an electrical musical instrument comprising means for generating a frequency control signal having a parameter directly related to frequency of a musical signal to be generated therefrom, means responsive to said frequency control signal for generating repetitive groups of amplitude sample representations collectively delineating a wave shape of a musical note, the repetition rate of said groups being directly controlled by said control signal, means for selecting a value of said parameter that uniquely defines a desired nominal frequency of a musical note, modulating means for changing the selected value of said control signal parameter, and means for converting said groups of amplitude sample representations to a musical signal of said nominal frequency as modulated by said modulating means.
 7. The system of claim 6 wherein said control signal is a digital representation of a number selected from a set of numbers, wherein said means for generating groups of amplitude sample representations comprises an address register, a control number register, and means for repetitively advancing the address register by the number in the control number register, and wherein said modulating mEans comprises means for changing the number in said control number register to a number not contained in said set of numbers.
 8. The frequency modulation system of claim 6 wherein said means for generating groups of amplitude sample representations comprises a memory storing representations that collectively delineate such wave shape and addressing means for reading out said stored representations at a group repetition rate that defines the nominal frequency of a wave shape to be generated, wherein said means for generating a frequency control signal comprises a number storage device and means for introducing into said number storage device a number that identifies a nominal musical signal to be generated, and wherein said modulating means comprises means for changing at least one digit of said number in said number storage device.
 9. The frequency modulation system of claim 8 wherein said modulating means comprises means for changing the digit of a single order of the number in said number storage device by a selected amount for any one of a plurality of numbers that may be contained in said number storage device.
 10. A musical tone generation system comprising a memory containing representations of sampled amplitudes of a wave form of a musical note, memory address means for repetitively reading said representations from said memory to provide an output comprising repetitive groups of said representations, said groups having repetition rate corresponding to the frequency of the musical tone to the frequency of the musical tone to be generated thereby, a number storage device and means for repetitively augmenting said memory address means by the amount of the number stored in said number storage device, thereby to control the repetition rate of groups of representations read from said memory, means for selecting the frequency of a note to be generated, means responsive to said note frequency selecting means for introducing into said number storage device a number selected from a set of numbers that represent notes of a musical scale, and frequency modulation means for changing the number in said number storage device as representations are being read from said memory.
 11. The tone generation system of claim 10 wherein said frequency modulation means comprises means for changing the digit stored in a preselected order of said number storage device.
 12. In a musical tone generation system comprising a memory containing representations of sampled amplitudes of a wave form of a musical note, memory address register means for repetitively reading said representations from said memory to provide an output comprising repetitive groups of said representations, said groups having a repetition rate corresponding to the frequency of the musical tone to be generated thereby, address rate control means for controlling said address register to thereby control the read out repetition rate of said groups read from said storage memory, and manually selectable means for controlling said address rate control means so as to select a desired musical tone of a musical scale, the improvement comprising means for modulating said address rate control means so as to modulate the frequency of the selected musical tone.
 13. The system of claim 12 wherein said address rate control means is a number storage register, and wherein said modulating means comprises means for changing the digit in one order of low significance of said number storage register. 